I don’t want to be spending hundreds and hundreds of pounds on one, any suggestions (and keep it simple) would be appreciated.
I would suggest you decide on your budget first. All the mash tuns you see are capable of producing good beer.
For very little money you can use an adapted cool box type. You get your wort to the desired temperature, add it to the tun, and the insulation of the vessel holds the temperature.
You can also use a very big pot, which you heat up on a stove or burner, and use a thermometer to keep checking the temperature.
Moving up, you can get an electrically heated vessel with a thermostat. You stir every so often to even out the temperature distribution. You can get these in plastic or stainless steel. Eg electrim do one for less than £100. The same thing in stainless steel costs more but you may find it nicer to deal with.
Moving up again you get stainless steel vessels with a heater, thermostat and a grain basket or removable false bottom. Sometimes they are called 'all in one' vessels, but they aren't all in one they are two in one. This is because you can do the mash AND boil the hops in the same vessel. (You can do the same thing with a cheaper vessel by adding the grain in a bag).
Moving up again, you get to all in one vessels that have recirculation. The wort is pumped round while mashing, keeling the temperature even. These typically cost upwards of £300.
The cheaper systems are manual - eg you set the temperature you want, and if you want to chsnge the temoerature later in the mash, you change it then.
The more expensive systems (typically over £400) are programmable, and usually can be controlled by an app via wifi or bluetooth. Brewzilla and Grainfather are 2 of the names you may have seen. The Grainfather G40 costs about a grand.
Whether a £1000 system produces better beer than a £50 system is largely down to you!
Cheers
